A flat panel display device (FPD) such as an organic EL (electro-luminescence) display (OELD), a plasma display panel (PDP) or a liquid crystal display device (LCD) has such a structure that a glass substrate for an element having a light-emitting element formed and a glass substrate for sealing are disposed to face each other and the light-emitting element is sealed in a glass package comprising two such glass substrates bonded (Patent Document 1). Further, for a solar cell such as a dye-sensitized solar cell, application of a glass package having a solar cell element (photoelectric conversion element) sealed with two glass substrates has been studied (Patent Document 2).
As a sealing material to seal a space between two glass substrates, a sealing resin or a sealing glass has been used. Since an organic EL (OEL) element or the like is likely to undergo deterioration by water, application of sealing glass excellent in the moisture resistance, etc. is in progress. Since the sealing temperature of the sealing glass is at a level of from 400 to 600° C., properties of an electronic element portion of the OEL element or the like will be deteriorated when firing is conducted by using a conventional heating furnace. Accordingly, it has been attempted that a layer of a glass material for sealing containing a laser absorbent is disposed between sealing regions provided on the peripheral portions of two glass substrates, and the layer of the glass material for sealing is irradiated with a laser light to heat and melt the layer thereby to conduct sealing (Patent Documents 1 and 2).
Sealing by irradiation with laser (laser sealing) can suppress thermal influences over the electronic element portion, and on the contrary, it has a disadvantage that cracks, fractures and the like are likely to form on the glass substrate at the time of sealing. In a case where the laser sealing is applied, first, a glass material for sealing containing a laser absorbent is burnt on a sealing region of a glass substrate for sealing to form a layer of the glass material for sealing in a form of a frame. Then, the glass substrate for sealing and a glass substrate for an element are laminated via the layer of the glass material for sealing, and the laminate is irradiated with a laser light from the side of the glass substrate for sealing to heat and melt the entire layer of the glass material for sealing thereby to seal a space between the glass substrates.
The layer of the glass material for sealing in the form of a frame is irradiated with the laser light along it. That is, the layer of the glass material for sealing in the form of a frame is irradiated with the laser light over its perimeter by scanning the layer of the glass material for sealing along it with the laser light from the laser light irradiation starting point. Accordingly, the laser light irradiation completion point rides the irradiation starting point. In a case where the layer of a glass material for sealing in the form of a frame is irradiated with a laser light with scanning over the perimeter, the glass substrate is likely to have cracks, fracture and the like at the laser light irradiation completion point. Alkali-free glass or soda lime glass is used for a glass substrate constituting a glass panel, and particularly soda lime glass has a high thermal expansion coefficient, and accordingly cracks, fractures and the like are likely to form at the time of laser sealing.
As a technique to keep a constant distance between two glass substrates, Patent Document 3 discloses a sealing material having beads such that the maximum particle size and the minimum particle size are within ±20% of the average particle size, added to a glass powder. Patent Document 4 discloses a PDP having a space between glass substrates sealed by a sealing material containing nonporous bead spacers having a grain size within a range of five sixths to 1.5 times the distance between facing glass substrates. Both techniques employ a firing step employing a heating furnace for the sealing step, and the laser sealing is not considered. Further, Patent Document 3 discloses use of beads having a narrow particle size distribution relative to the average particle size, and Patent Document 4 discloses use of beads having a broad particle size distribution relative to the central particle size, and the proportion of particles to the sealing glass is not considered.